METHOD FOR PRODUCING B-Ga203 SUBSTRATE AND METHOD FOR PRODUCING CRYSTAL LAMINATE STRUCTURE

ABSTRACT

Provided are: a method for producing a β-Ga 2 O 3  substrate of which changes in donor concentration in a reducing atmosphere or an inert gas atmosphere are suppressed; and a method for producing a crystal laminate structure that can epitaxially grow a high-quality crystal film having low variability of quality in a reducing atmosphere or an inert gas atmosphere. The method for producing a β-Ga 2 O 3  substrate includes a step for cutting out a β-Ga 2 O 3  substrate from a β-Ga 2 O 3  crystal containing a group IV element; annealing processing in an atmosphere containing a reducing atmosphere and/or an inert gas atmosphere is performed on the β-Ga 2 O 3  crystal before cutting out the β-Ga 2 O 3  substrate, or on the cut-out β-Ga 2 O 3  substrate.

TECHNICAL FIELD

The invention relates to a method for producing β-Ga₂O₃ based substrate(herein a β-Ga₂O₃ substrate) and a method for producing a crystallaminate structure (or laminated crystal structure).

BACKGROUND ART

A method of controlling electrical resistivity of a β-Ga₂O₃ substrate byimplanting a dopant such as Si is known (see e.g. PTL 1).

CITATION LIST Patent Literature

-   [PTL 1]-   JP-A-2005-235961

SUMMARY OF INVENTION Technical Problem

However, when the β-Ga₂O₃ substrate containing a dopant is exposed to areduction atmosphere or an inert gas atmosphere for, e.g., epitaxialcrystal growth by a MOCVD (Metal Organic Chemical Vapor Deposition)method, there is a possibility that the substrate per se is reduced,causing an increase in donor concentration.

In addition, the increase in donor concentration causes change in lightabsorption properties mainly in a region on a long-wavelength side withrespect to the near-infrared, which results in that temperature of theβ-Ga₂O₃ substrate changes during epitaxial growth when a crystal isepitaxially grown by a method using radiation to heat, such as MOCVD.For epitaxial crystal growth, temperature of substrate is a veryimportant parameter which affects crystal quality. Therefore, whentemperature characteristics vary in accordance with variation in lightabsorption properties of the substrate, quality of crystal to be grownmay vary.

It is an object of the invention to provide a method for producing aβ-Ga₂O₃ substrate which suppresses a change in donor concentration in areduction atmosphere or an inert gas atmosphere, as well as a method forproducing a crystal laminate structure which can epitaxially grow ahigh-quality crystal film having low variability of quality in areduction atmosphere or an inert gas atmosphere.

Solution to Problem

According to one embodiment of the invention, a method for producing aβ-Ga₂O₃ based substrate as defined in [1] to [4] below and a method formanufacturing a crystal laminate structure as defined in [5] to [8]below are provided so as to achieve one of the above object.

[1] A method for producing a β-Ga₂O₃ based substrate, comprising a stepfor cutting out a β-Ga₂O₃ based substrate from a β-Ga₂O₃ based crystalcontaining a group IV element,

-   -   wherein annealing processing in an atmosphere comprising at        least one of a reduction atmosphere and an inert gas atmosphere        is performed on the 13-Ga₂O₃ based crystal before cutting out        the β-Ga₂O₃ based substrate or on the β-Ga₂O₃ based substrate.

[2] The method for producing a β-Ga₂O₃ based substrate according to [1],wherein the reduction atmosphere comprises an H₂ atmosphere.

[3] The method for producing a β-Ga₂O₃ based substrate according to [1]or [2], wherein the inert gas atmosphere comprises at least one of a N2atmosphere, an Ar atmosphere, a Ne atmosphere and a He atmosphere.

[4] The method for producing a β-Ga₂O₃ based substrate according to [1]or [2], wherein the group IV element comprises Si.

[5] A method for producing a crystal laminate structure, comprising:

-   -   a step for cutting out a β-Ga₂O₃ based substrate from aβ-Ga₂O₃        based crystal containing a group IV element; and    -   a step for epitaxially growing a crystal film on the β-Ga₂O₃        based substrate in a first atmosphere comprising at least one of        a first reduction atmosphere and a first inert gas atmosphere,    -   wherein annealing processing in a second atmosphere comprising        at least one of a second reduction atmosphere and a second inert        gas atmosphere is performed on the β-Ga₂O₃ based crystal before        cutting out the β-Ga₂O₃ based substrate or on the β-Ga₂O₃ based        substrate before epitaxially growing the crystal film.

[6] The method for producing a crystal laminate structure according to[5], wherein the first and second reduction atmospheres comprise an H₂atmosphere.

[7] The method for producing a crystal laminate structure according to[5] or [6], wherein the first and second inert gas atmospheres compriseat least one of a N₂ atmosphere, an Ar atmosphere, a Ne atmosphere and aHe atmosphere.

[8] The method for producing a crystal laminate structure according to[5] or [6], wherein the group IV element comprises Si.

Advantageous Effects of Invention

According to one embodiment of the invention, a method for producing aβ-Ga₂O₃ substrate can be provided which suppresses a change in donorconcentration in a reduction atmosphere or an inert gas atmosphere, aswell as a method for producing a crystal laminate structure which canepitaxially grow a high-quality crystal film having low variability ofquality in a reduction atmosphere or an inert gas atmosphere.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing a relation between a Si concentration and adonor concentration in a β-Ga₂O₃ crystal (herein a β-Ga₂O₃ basedcrystal) before and after annealing processing.

FIG. 2 is a graph showing a relation between a donor concentration andlight absorption properties of a β-Ga₂O₃ substrate.

FIG. 3 is a vertical cross sectional view showing an example of acrystal laminate structure.

FIG. 4 is a graph showing variation in the donor concentration inβ-Ga₂O₃ substrates with and without annealing processing and before andafter epitaxial crystal growth.

FIG. 5A is a photograph showing a state of an annealed β-Ga₂O₃ substrateafter epitaxial crystal growth.

FIG. 5B is a photograph showing a state of a non-annealed β-Ga₂O₃substrate after epitaxial crystal growth.

FIG. 6 is a graph showing a relation between a donor concentration andresistivity of the β-Ga₂O₃ substrate.

DESCRIPTION OF EMBODIMENTS Embodiment

One of the essential points of the present embodiment is that a donorconcentration in a β-Ga₂O₃ substrate is preliminarily increased prior toa process performed in an atmosphere including at least one of areduction atmosphere and an inert gas atmosphere, e.g., epitaxialcrystal growth on the β-Ga₂O₃ substrate, to solve problems caused by anincrease in a donor concentration during such a process. A specificexample of the embodiment will be described below.

(Manufacture of β-Ga₂O₃ substrate)

Manufacture of a β-Ga₂O₃ substrate will be described below. Sincegallium oxide is transparent and conductive, the β-Ga₂O₃ substrate isuseful as a substrate of a light-emitting element having a verticalelectrode structure and has attracted attention in recent years.

Firstly, a β-Ga₂O₃ crystal containing Si as a dopant is formed by acrystal growth method such as EFG (Edge-defined film-fed growth) methodor FZ (Floating Zone) method. The Si concentration in the β-Ga₂O₃crystal is controlled according to the desired electrical resistivity ofthe β-Ga₂O₃ substrate.

In case that the β-Ga₂O₃ crystal is grown by the EFG method, forexample, Ga₂O₃ powder and SiO₂ powder which is a Si material as a dopantare melted and the resulting melt is drawn up using a seed crystal togrow a crystal, thereby obtaining a plate-like β-Ga₂O₃ crystal. In caseof using the FZ method, for example, a feed rod made of Ga₂O₃ powder andSiO₂ powder which is a Si material as a dopant is vertically held and ispartially heated to make a molten zone, and a crystal is grown by movingthe molten zone upward or downward while holding by surface tension,thereby obtaining a column-shaped β-Ga₂O₃ crystal.

The β-Ga₂O₃ crystal is a β-Ga₂O₃ single crystal or a β-Ga₂O₃ singlecrystal with an element such as Al or In added thereto, and contains Sias a dopant.

At this stage, the donor concentration is often lower than the Siconcentration in the grown β-Ga₂O₃ crystal and the donor concentrationwith respect to the Si concentration varies depending on the growncrystal. Thus, the grown β-Ga₂O₃ crystal is subjected to annealingprocessing in an atmosphere including at least one of a reductionatmosphere and an inert gas atmosphere to bring the donor concentrationin the grown β-Ga₂O₃ crystal close to the Si concentration in the grownβ-Ga₂O₃ crystal, thereby reducing variation in the donor concentrationwith respect to the Si concentration.

The reduction atmosphere used for this annealing processing is, e.g., aH₂ atmosphere. Meanwhile, the inert gas atmosphere is, e.g., a N₂atmosphere, an Ar atmosphere, a Ne atmosphere, a He atmosphere or amixed atmosphere including two or more thereof. Annealing treatmenttemperature is, e.g., not less than 800° C. and not more than 1725° C.which is a melting point of gallium oxide.

FIG. 1 is a graph showing a relation between a Si concentration and adonor concentration in a β-Ga₂O₃ crystal before and after theabove-mentioned annealing processing. In FIG. 1, the vertical axisindicates a donor concentration (/cm³) and the horizontal axis indicatesa Si concentration (atoms/cm³). Open triangles and filled circles inFIG. 1 respectively indicate measured values before annealing processingand those after annealing processing.

To obtain the measured values shown in FIG. 1, plural β-Ga₂O₃ crystalshaving different Si concentrations were prepared and a donorconcentration of each crystal was measured before and after annealingprocessing. In the annealing processing, the temperature was increasedto 1000° C. in 19 minutes in an atmosphere consisting 100% of N₂, wasthen kept at 1000° C. for 1 hour and was lowered to room temperature in19 minutes. Here, the donor concentration was measured using a C—Vmeasurement device and the Si concentration was measured by SIMSanalysis.

Before annealing processing, some β-Ga₂O₃ crystals have a largedifference between the Si concentration and the donor concentration andthe donor concentration with respect to the Si concentration variesgreatly, as shown in FIG. 1. On the other hand, after annealingprocessing, the donor concentration is close to the Si concentration inall β-Ga₂O₃ crystals and variation in the donor concentration withrespect to the Si concentration is reduced.

After that, a β-Ga₂O₃ substrate is cut out from the plate-like orcolumn-shaped β-Ga₂O₃ crystal. It should be noted that theabove-mentioned annealing processing may be performed after a β-Ga₂O₃substrate is cut out from the O-Ga₂O₃ crystal. Alternatively, theabove-mentioned annealing processing may be performed on a β-Ga₂O₃substrate after a polishing process.

In the β-Ga₂O₃ crystal, the donor concentration correlates with lightabsorption properties in a region on a long-wavelength side with respectto the near-infrared. Therefore, when each β-Ga₂O₃ crystal has asignificantly different donor concentration, light absorption propertiesthereof are also significantly different. In such a case, substratetemperature during crystal growth is different in each substrate when,for example, a crystal film is epitaxially grown on the β-Ga₂O₃substrate cut out from the β-Ga₂O₃ crystal and this may cause variationin quality of an epitaxial crystal film.

FIG. 2 is a graph showing a relation between a donor concentration andlight absorption properties of a β-Ga₂O₃ substrate. In FIG. 2, thevertical axis indicates light absorption coefficient of theβ-Ga₂O₃substrate at a wavelength of 750 nm and the horizontal axis indicates adonor concentration (/cm³).

In the β-Ga₂O₃ substrate, the donor concentration is substantiallyproportional to the light absorption coefficient at a wavelength of 750nm as shown in FIG. 2, and the light absorption coefficient increaseswith an increase in the donor concentration.

(Manufacture of Crystal Laminate Structure)

The β-Ga₂O₃ substrate is formed and a crystal film is subsequentlyepitaxially grown on the β-Ga₂O₃ substrate, thereby forming a crystallaminate structure including the β-Ga₂O₃ substrate and an epitaxialcrystal film.

For example, a GaN-based crystal film is epitaxially grown on theβ-Ga₂O₃ substrate by the MOCVD method. In the MOCVD method, a crystalgrows in a reduction atmosphere such as hydrogen atmosphere, ammoniaatmosphere or a mixed atmosphere of hydrogen and ammonia. In the presentembodiment, the substrate temperature during crystal growth hardlychanges since the donor concentration in the β-Ga₂O₃ substrate ispreliminarily increased by the above-mentioned annealing processing andit is thus possible to form a high-quality epitaxial crystal film withless variation in quality.

FIG. 3 is a vertical cross sectional view showing an example of acrystal laminate structure in the present embodiment. A crystal laminatestructure 1 has a β-Ga₂O₃ based crystal film 2 and an epitaxial crystalfilm 3 on the β-Ga₂O₃ crystal film 2.

FIG. 4 is a graph showing variation in the donor concentration inβ-Ga₂O₃ substrates with and without annealing processing and before andafter exposure to epitaxial crystal growth atmosphere. In FIG. 4, thevertical axis indicates a donor concentration (/cm³), the left side ofthe horizontal axis indicates before exposure to epitaxial crystalgrowth atmosphere and the right side of the horizontal axis indicatesafter exposure to epitaxial crystal growth atmosphere. In FIG. 4, filledcircles are measured values of an annealed β-Ga₂O₃ substrate in thepresent embodiment, and open triangles, open diamonds and open squaresare measured values of non-annealed β-Ga₂O₃ substrates.

To obtain the measured values shown in FIG. 4, one β-Ga₂O₃ substrateobtained from an annealed β-Ga₂O₃ crystal and three β-Ga₂O₃ substratesobtained from on-annealed β-Ga₂O₃ crystals were prepared and a donorconcentration of each substrate was measured before and after exposureto epitaxial crystal growth atmosphere. All of the four β-Ga₂O₃substrates have a Si concentration of about 7.5×10¹⁸/cm³. In theannealing processing performed on the one β-Ga₂O₃ crystal, thetemperature was increased to 1450° C. in 9 hours in a N₂ atmosphere, wasthen kept at 1450° C. for 6 hours and was lowered to room temperature in12 hours.

As shown in FIG. 4, the donor concentration in the annealed β-Ga₂O₃substrate hardly changes before and after exposure to epitaxial crystalgrowth atmosphere. This is because the donor concentration in theβ-Ga₂O₃ substrate is preliminarily increased by annealing processing andis close to the Si concentration in the β-Ga₂O₃ substrate.

On the other hand, the donor concentration before epitaxial crystalgrowth is significantly different between the non-annealed β-Ga₂O₃substrates and, in the substrates having a low donor concentration, thedonor concentration increases toward the Si concentration from before toafter exposure to epitaxial crystal growth atmosphere. Due tosignificant difference in the donor concentration before exposure toepitaxial crystal growth atmosphere, the degree of change in substratetemperature during crystal growth is different in each substrate andquality of epitaxially-grown crystal is likely to be different in eachsubstrate.

FIG. 5A is a photograph showing a state of the annealed β-Ga₂O₃substrate after epitaxial crystal growth and FIG. 5B is a photographshowing a state of the non-annealed β-Ga₂O₃ substrate after epitaxialcrystal growth. A crystal on aβ-Ga₂O₃ substrate shown in FIG. 5A and acrystal on β-Ga₂O₃ substrate shown in FIG. 5B were epitaxially grownunder the same conditions.

As shown in FIG. 5B, a crystal film is separated from the non-annealedβ-Ga₂O₃ substrate (in a region on the left side of the substrate). Thereason for this is considered that a good quality crystal was notobtained due to change in the substrate temperature during epitaxialcrystal growth. On the other hand, the epitaxially-grown film on theannealed β-Ga₂O₃ substrate is not separated as shown in FIG. 5A and itis shown that a good quality crystal is obtained.

Other than in the case of epitaxial crystal growth, a problem due tovariations in the donor concentration and light absorption properties ofthe β-Ga₂O₃ substrate may also occur when a process in an atmosphereincluding at least one of a reduction atmosphere and an inert gasatmosphere is performed on a β-Ga₂O₃ substrate which is not subjected tothe annealing processing of the present embodiment.

EFFECTS OF THE EMBODIMENT

In the present embodiment, annealing processing in an atmosphereincluding at least one of a reduction atmosphere and an inert gasatmosphere enables to obtain a β-Ga₂O₃ substrate in which variation inthe donor concentration and resulting variation in light absorptionproperties in a region on a long-wavelength side with respect to thenear-infrared are suppressed.

In addition, in such a β-Ga₂O₃ substrate, change in the donorconcentration in an atmosphere including at least one of a reductionatmosphere and an inert gas atmosphere is small. Thus, when a crystalfilm is epitaxially grown on the β-Ga₂O₃ substrate, an epitaxial crystalfilm with small variation in quality is formed and it is thus possibleto obtain a high-quality crystal laminate structure.

Furthermore, other than in the case of epitaxial crystal growth, it ispossible to suppress the problem due to variations in the donorconcentration and light absorption properties of the β-Ga₂O₃ substratealso when a process in an atmosphere including at least one of areduction atmosphere and an inert gas atmosphere is performed on theβ-Ga₂O₃ substrate in the present embodiment. In addition, for example,when a device is formed to include the β-Ga₂O₃ substrate or the crystallaminate structure in the present embodiment, it is possible to obtain ahigh-performance device with small variation in electric characteristicsand optical characteristics.

It should be noted that it is possible to control electrical resistanceof the β-Ga₂O₃ substrate by changing the donor concentration. Theβ-Ga₂O₃ substrate can be used as a part of current path and thus can beused as a substrate of a light-emitting element having a verticalelectrode structure. FIG. 6 is a graph showing a relation between adonor concentration and resistivity of the β-Ga₂O₃ substrate. Theelectrical resistivity here is electrical resistivity in a thicknessdirection which is measured between electrodes respectively connected tofront and back sides of the β-Ga₂O₃ substrate. As shown in FIG. 6,resistivity decreases with an increase in the donor concentration.

Although Si is used as a dopant to a β-Ga₂O₃ crystal in theabove-mentioned embodiment, another group IV element such as Si, Hf, Ge,Sn, Ti or Zr may be used. In addition, two or more types of group IVelements may be used.

Although the embodiment of the invention has been described above, theinvention according to claims is not to be limited to theabove-mentioned embodiment. Further, it should be noted that allcombinations of the features described in the embodiment are notnecessary to solve the problem of the invention.

INDUSTRIAL APPLICABILITY

Provided are a method for producing a β-Ga₂O₃ substrate of which changesin donor concentration in a reduction atmosphere or an inert gasatmosphere are suppressed, and a method for producing a crystal laminatestructure which can epitaxially grow a high-quality crystal film havinglow variability of quality in a reduction atmosphere or an inert gasatmosphere.

REFERENCE SIGNS LIST

-   1 crystal laminate structure-   2 Ga₂O₃ based substrate-   3 epitaxial crystal film

1. A method for producing a β-Ga₂O₃ based substrate, comprising cuttingout a β-Ga₂O₃ based substrate from a β-Ga₂O₃ based crystal containing agroup IV element, wherein annealing processing in an atmospherecomprising at least one of a reduction atmosphere and an inert gasatmosphere is performed on the β-Ga₂O₃ based crystal before cutting outthe β-Ga₂O₃ based substrate or on the β-Ga₂O₃ based substrate.
 2. Themethod for producing a β-Ga₂O₃ based substrate according to claim 1,wherein the reduction atmosphere comprises an H₂ atmosphere.
 3. Themethod for producing a β-Ga₂O₃ based substrate according to claim 1,wherein the inert gas atmosphere comprises at least one of a N₂atmosphere, an Ar atmosphere, a Ne atmosphere and a He atmosphere. 4.The method for producing a β-Ga₂O₃ based substrate according to claim 1,wherein the group IV element comprises Si.
 5. A method for producing acrystal laminate structure, comprising: cutting out a β-Ga₂O₃ basedsubstrate from a β-Ga₂O₃ based crystal containing a group IV element;and epitaxially growing a crystal film on the β-Ga₂O₃ based substrate ina first atmosphere comprising at least one of a first reductionatmosphere and a first inert gas atmosphere, wherein annealingprocessing in a second atmosphere comprising at least one of a secondreduction atmosphere and a second inert gas atmosphere is performed onthe β-Ga₂O₃ based crystal before cutting out the β-Ga₂O₃ based substrateor on the β-Ga₂O₃ based substrate before epitaxially growing the crystalfilm.
 6. The method for producing a crystal laminate structure accordingto claim 5, wherein the first and second reduction atmospheres comprisean H₂ atmosphere.
 7. The method for producing a crystal laminatestructure according to claim 5, wherein the first and second inert gasatmospheres comprise at least one of a N₂ atmosphere, an Ar atmosphere,a Ne atmosphere and a He atmosphere.
 8. The method for producing acrystal laminate structure according to claim 5, wherein the group IVelement comprises Si.
 9. The method for producing a β-Ga₂O₃ basedsubstrate according to claim 2, wherein the inert gas atmospherecomprises at least one of a N₂ atmosphere, an Ar atmosphere, a Neatmosphere and a He atmosphere.
 10. The method for producing a β-Ga₂O₃based substrate according to claim 2, wherein the group IV elementcomprises Si.
 11. The method for producing a crystal laminate structureaccording to claim 6, wherein the first and second inert gas atmospherescomprise at least one of a N₂ atmosphere, an Ar atmosphere, a Neatmosphere and a He atmosphere.
 12. The method for producing a crystallaminate structure according to claim 6, wherein the group IV elementcomprises Si.